Expansible thermoplastic resin particles may be made by first conducting a polymerization and then an impregnation of the resin particles with expansion agent. The impregnated particles are molded into foamed articles by partially filling molds and heating until the resin particles expand, fuse and cool to form the final object. Particularly when the polymer is polystyrene and the impregnated particles are expansible beads of polystyrene, there is great difficulty in satisfactorily conducting the polymerizations to give narrow size distributions of polystyrene particles and to separate these distributions by size classification into predetermined fractions which upon impregnation give beads of uniform size and properties for optimum performance in subsequent molding operations. These same problems also exist when it is desired to obtain selected fractions of polymer beads for subsequent processes such as seed polymerizations.